Spin gap and resonance at the nesting wave vector in superconducting FeSe0.4Te0.6

Neutron scattering is used to probe magnetic excitations in FeSe0.4Te0.6 (Tc=14K). Low energy spin fluctuations are found with a characteristic wave vector (1212L) that corresponds to Fermi surface nesting and differs from Qm=(δ012) for magnetic ordering in Fe1+yTe. A spin resonance with Ω0=6.51(4)meV 5.3kBTc and Γ=1.25(5)meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s± superconductivity and imperfect quasi-2D Fermi surface nesting.